This invention is concerned with coating bituminous substrates. More particularly, though not exclusively, this invention is concerned with a method for improving the quality of a mastic coating system.
A large proportion of the roofing market utilizes asphalt-based products, for example, modified bituminous rolls, as a base substrate which is then topcoated with conventional roof mastics to improve durability (asphalt degradation), provide energy savings (white coating vs. black asphalt), reduce the fire rating and improve aesthetics. Such coatings are frequently referred to as xe2x80x9croof masticsxe2x80x9d although xe2x80x9cmastic coatingsxe2x80x9d as used herein includes any such thick coatings applied to a substantially horizontal surface such as, for example, a roof deck, to a substantially vertical surface such as, for example, a wall, or to other substrate surfaces. Unfortunately, these mastic coatings have deficiencies. A roof deck, or other surfaces, by design or because of imperfections, may retain water. This ponded water results in loss of adhesion and blistering leading to failure of the mastic coating. There is also a tendency for mastic coatings to become discolored by the bleedthrough of black chemicals from the asphaltic substrate on which the mastic is applied.
A mastic coating system which displays good adhesion and superior resistance to blistering especially when exposed to ponded water is disclosed in U.S. Pat. No. 5,059,456. This system relies upon the use of a water-based tiecoat between the substrate and the mastic coating. The tiecoat consists of a water-insoluble latex polymer and multivalent metal ion, wherein the latex polymer comprises units such as would result from preparation from a monomer mixture comprising at least 20 weight %, based on the weight of the monomer mixture, of at least one hydrophobic monomer selected from the group consisting of (C4-C20)-alkyl methacrylates and (C8-C20)-alkyl acrylates, and from 3.0 weight % to 7.5 weight % of methacrylic acid, based on the weight of the monomer mixture, and where the latex polymer has a glass transition temperature of from xe2x88x9220xc2x0 C. to 5xc2x0 C., and where the molar ratio of the multivalent metal ion to the methacrylic acid in the latex polymer is from about 1:1 to about 0.375:1. The tiecoat is applied to a substrate before subsequent application of a mastic coating. Though this system offers properties of adhesion and resistance to blistering which are superior to conventional mastic systems, there remains a desire to improve upon these properties.
It is an object of the present invention to provide a mastic coating system which offers improved adhesion and resistance to blistering over those described in U.S. Pat. No. 5,059,456. Moreover, it is preferred if the above improvements in the quality of a mastic coating system can be achieved with a system which does not essentially rely upon the use of a tiecoat between the substrate and mastic composition.
In accordance with the present invention, there is provided a method for improving the quality of a mastic coating system comprising:
(A) forming a mastic topcoat composition and optionally a tiecoat composition;
(B) optionally applying said tiecoat composition onto a bituminous substrate;
(C) applying said mastic topcoat composition either onto a bituminous substrate directly or, when present, onto said tiecoat composition which has already been applied onto said bituminous substrate;
wherein at least one of said mastic topcoat composition and said optional tiecoat composition comprises a water-insoluble latex polymer;
wherein said latex polymer has a Tg of from xe2x88x9225xc2x0 C. to 20xc2x0 C., preferably from xe2x88x9210xc2x0 C. to 0xc2x0 C., and is prepared from a monomer mixture comprising:
(a) at least 20%, preferably from 30 to 70%, by weight, of at least one hydrophobic monomer selected from the group consisting of (C8 to C24) alkyl (meth)acrylates;
(b) 0 or up to 45%, preferably 0 or up to 35%, more preferably 0 or up to 10%, by weight of at least one vinyl aromatic monomer;
(c) 0 or up to 75%, preferably up to 74.5%, more preferably 5 to 70%, even more preferably 5 to 50%, by weight of at least one monomer selected from the group consisting of (C1 to C4) alkyl (meth)acrylates; and
(d) 0 or up to 5%, preferably from 0.5 to 2.5%, by weight of at least one monomer selected from acrylic or methacrylic acid;
provided that when (b) is not present in the mixture the amount of (d) is in the range from 0 or up to less than 3%, preferably 0.05 to less than 3% by weight; and wherein the weight of (a) and (b) together constitute at least 25%, preferably 30% or more, more preferably 35% or more, even more preferably 45% or more, by weight of said monomer mix.
In another aspect, the present invention provides a composition suitable for use as a mastic topcoat composition or a mastic tiecoat composition comprising a water-insoluble latex polymer; wherein said latex polymer has a Tg of from xe2x88x9225xc2x0 C. to 20xc2x0 C., preferably from xe2x88x9210xc2x0 C. to 0xc2x0 C., and is prepared from a monomer mixture comprising:
(a) at least 20%, preferably from 30 to 70%, by weight, of at least one hydrophobic monomer selected from the group consisting of (C8 to C24) alkyl (meth)acrylates;
(b) 0 or up to 45%, preferably 0 or up to 35%, more preferably 0 or up to 10%, by weight of at least one vinyl aromatic monomer;
(c) 0 or up to 75%, preferably up to 74.5%, more preferably 5 to 70%, even more preferably 5 to 50%, by weight of at least one monomer selected from the group consisting of (C1 to C4) alkyl (meth)acrylates; and
(d) 0 or up to 5%, preferably from 0.5 to 2.5%, by weight of at least one monomer selected from acrylic or methacrylic acid;
provided that when (b) is not present in the mixture the amount of (d) is in the range from 0 or up to less than 3%, preferably 0.05 to less than 3% by weight; and wherein the weight of (a) and (b) together constitute at least 25%, preferably 30% or more, more preferably 35% or more, even more preferably 45% or more, by weight of said monomer mix.
In yet another aspect, the present invention provides the use in a mastic coating system to improve adhesion and resistance to blistering of a mastic topcoat of a water-insoluble latex polymer; wherein said latex polymer has a Tg of from xe2x88x9225xc2x0 C. to 20xc2x0 C., preferably from xe2x88x9210xc2x0 C. to 0xc2x0 C., and is prepared from a monomer mixture comprising:
(a) at least 20%, preferably from 30 to 70%, by weight, of at least one hydrophobic monomer selected from the group consisting of (C8 to C24) alkyl (meth)acrylates;
(b) 0 or up to 45%, preferably 0 or up to 35%, more preferably 0 or up to 10%, by weight of at least one vinyl aromatic monomer;
(c) 0 or up to 75%, preferably up to 74.5%, more preferably 5 to 70%, even more preferably 5 to 50%, by weight of at least one monomer selected from the group consisting of (C1 to C4) alkyl (meth)acrylates; and
(d) 0 or up to 5%, preferably from 0.5 to 2.5%, by weight of at least one monomer selected from acrylic or methacrylic acid;
provided that when (b) is not present in the mixture the amount of (d) is in the range from 0 or up to less than 3%, preferably 0.05 to less than 3% by weight; and wherein the weight of (a) and (b) together constitute at least 25%, preferably 30% or more, more preferably 35% or more, even more preferably 45% or more, by weight of said monomer mix.
Preferably at least one of (b) and (d) is present in the monomer mixture.
Mastic coating systems employing the topcoat and/or tiecoat of this invention display surprisingly improved adhesion and resistance to blistering, especially when exposed to ponded water. They are also resistant to bleedthrough of dark chemicals from the bituminous substrate.
A common substrate for application of the invention is modified bituminous sheet.
The latex polymer can be prepared by emulsion polymerization techniques well known in the art. For example, U.S. Pat. No. 5,521,266 discloses a very suitable process of aqueous polymerization suitable for forming polymers containing monomers having low water solubility.
The at least one hydrophobic monomer (a) is preferably selected from the group consisting of ethylhexyl acrylate (EHA), octyl methacrylate, isooctyl methacrylate, decyl methacrylate (n-DMA), isodecyl methacrylate (IDMA), lauryl methacrylate (LMA), pentadecyl methacrylate, stearyl methacrylate (SMA), octyl acrylate, isooctyl acrylate, decyl acrylate, isodecyl acrylate, lauryl acrylate (LA) and the (C12 to C15) alkyl methacrylates. LMA, IDMA and the (C12 to C15) alkyl methacrylates are the most preferred monomers.
The at least one hydrophobic monomer (a) is most preferably present in the monomer mix in an amount of from 30 to 70% by weight.
The at least one vinyl aromatic monomer is preferably selected from the group consisting of styrene (Sty), vinyl toluene, 2-bromo styrene, o-bromo styrene, p-chloro styrene, o-methoxy styrene, p-methoxy styrene, allyl phenyl ether, allyl tolyl ether and alpha-methyl styrene. Styrene is the most preferred monomer.
The at least one vinyl aromatic monomer is preferably present in the monomer mix in an amount of less than 35%, more preferably 0 or up to 10%, by weight.
The preferred (C1 to C4) alkyl (meth)acrylates are methyl methacrylate (MMA), ethyl acrylate (EA), and butyl acrylate (BA).
The monomer mixture used to prepare the latex polymer preferably contains from about 0.5 wt. % to less than 3% (based on the weight of total monomers) of methacrylic acid. A preferred level of methacrylic acid is from 1 to 2%.
xe2x80x9cGlass transition temperaturexe2x80x9d, or xe2x80x9cTgxe2x80x9d, used herein means the mid-point glass transition temperature of a polymer as determined by differential scanning calorimetry, in accordance with ASTM E-1356-91.
The monomer components of the latex polymer should be selected such that the Tg of the dried latex polymer is from xe2x88x9225xc2x0 C. to +20xc2x0 C., and a Tg of xe2x88x9210xc2x0 C. to 0xc2x0 C. is preferred. Polymers having Tg""s above +5xc2x0 C. yield coatings which may lose their flexibility at low temperature. Polymers having Tg""s below xe2x88x9225xc2x0 C. are prone to bleed-through wherein the colored chemicals in the substrate migrate into the topcoat and tiecoat and deteriorate the performance of the mastic coating system.
The molecular weight of the latex polymer influences blister resistance and adhesion. Reducing the molecular weight of the polymer improves blister resistance and adhesion. Preferably, the latex polymer has a weight average molecular weight from about 10,000 to about 150,000 gives the best resistance to blistering and adhesion. Molecular weight can be controlled by a wide variety of chain transfer agents, as is well known to those skilled in the art. These include, for example, alkyl mercaptans, halogen compounds, and other well-known agents. A chain transfer agent such as, for example, n-dodecyl mercaptan (n-DDM), used at the level of about 0.1 wt. % (based on the weight of total monomers) to about 2.5 wt. % may be effective in providing the desired molecular weight. Weight average molecular weight (Mw) is determined by aqueous gel permeation chromatography.
Mastic topcoat compositions and tiecoat compositions of the present invention will typically comprise, in addition to the latex polymer, at least one or more of the following components: pigments, extenders, dispersants, surfactants, coalescents, wetting agents, thickeners, rheology modifiers, drying retarders, plasticizers, biocides, mildewicides, defoamers, colorants, waxes, adhesion promoters, zinc oxide and solid silica. Said compositions may also comprise one or more other latex polymers which substitute in part the water-insoluble latex polymer defined above.
Mastic topcoat compositions of the present invention will always comprise a pigment, such as for example titanium dioxide or Ropaque opaque polymer from Rohm and Haas Company.
Mastic topcoat and tiecoat compositions of the present invention will usually comprise a biocide and/or mildewicide, such as Kathon 887 or Skane M-8 from Rohm and Haas Company.
Tiecoat compositions of the present invention based on latex polymers having Tg""s of about xe2x88x9225xc2x0 C. to about +5xc2x0 C. are not sufficiently tack free at the temperatures at which roofing materials are normally applied. Therefore, it is preferable to incorporate into said tiecoat compositions a compound, such as a multivalent metal ion complex, to achieve a tack-free coating. Multivalent metal ions such as calcium, magnesium, zinc, barium and strontium ions, may be used. Complexes of multivalent metal ions, such as zinc hexammonia and the like, and salts of multivalent metal ions with counterions, such as chloride, acetate, bicarbonate and the like, may be used to supply the ions. Zinc is the preferred multivalent metal ion. The level of multivalent metal ion in the tiecoat composition can be controlled to achieve a tack-free coating and this is accomplished by controlling the molar ratio of added multivalent metal ion to methacrylic acid in the polymer tiecoat. Molar ratios of multivalent metal ion/methacrylic acid as low as about 0.375:1 and as high as about 1:1 can be used. A molar ratio of about 0.5:1 is preferred.
The following Examples, including Comparative Examples, are given solely for the purpose of illustrating details of the invention and are in no way limiting.